docs: def cull_nodes(self, stat, threshold=0.5, comparator=ge):
          """Delete nodes whose stat >= ``threshold`` (default 0.5).
  
          Optional argument ``comparator`` will replace >= as the test
          for whether to cull. You can use the name of a stored function.
  
          """
          comparator = self._lookup_comparator(comparator)
          dead = [
              name for name, node in self.node.items()
              if stat in node and comparator(node[stat], threshold)
          ]
          self.remove_nodes_from(dead)
          return self